An electrographic printing and copying, a charge pattern is created on a dielectric surface to form a latent image which is then developed by contact of the latent image with toner which is usually a powder or liquid. The dielectric surface may be a coating on a sheet or web of paper to which the toner is applied. Alternatively, the dielectric surface may be the charge retentive surface of a drum, belt or the like from which toner is transferred onto a sheet or web of paper.
An electrostatic latent image may be established through electrostatic induction by a charged writing head, by ion projection, or through photoconduction. In the cases of ion projection and photoconduction, formation of the latent image is provided by members which are spaced apart from the dielectric surface on which the image is formed. On the other hand, a charged writing head, as described in the U.S. Pat. No. 4,706,605 to Bibl et al., is in direct physical contact with the dielectric surface. This patent features a drum printer and a writing head comprising a linear array of closely spaced charging elements for writing an electrostatic latent image on a sheet mounted on a drum. In comparison, U.S. Pat. No. 4,198,923 to Blumenthal teaches a web of paper which is pressed against a print head. The benefit of a web system is that it is possible to provide a wrap angle around a large radius head so that the tension of the paper provides the contact with the head, much like magnetic tape on a magnetic head. Thus, the pressure is typically low compared to that of drum systems where a head must be pressed against a rotating drum. There are, however, advantages associated with the drum system. Among the chief benefits achieved in the drum printer is the fact that the sheet being printed upon is fully stabilized and accurately positioned by the drum during the printing process. As a result, accuracy and color registration are superior compared to the web system.
Physical contact of the writing head with the special coating of an electrostatic sheet may result in damage being done by the electrostatic sheet to the writing head, or damage done by the writing head to the electrostatic sheet. The problems are minor ones, but are annoying because they can affect print quality. Sometimes the coating of electrostatic paper, vellum, or film, is a relatively abrasive coating which may scratch the writing head and cause image noise. Other coatings may leave a resin deposit on the nibs, i.e. the working end of the charging elements, thereby causing some signal dropout. The writing head may also damage the electrostatic sheet by abrading the surface of the dielectric coating, sometimes causing background staining by the toners.
An object of the present invention is to provide a writing head for electrostatic image formation which significantly reduces the chance of head damage to an image carrier, and significantly reduces the chance of image carrier damage to the writing head.